As semiconductor device manufacturers continue to produce smaller devices, the requirements for photomasks used in the fabrication of these devices continue to tighten. Photomasks, also known as reticles or masks, typically consist of substrates that have an absorber layer formed on the substrate. The absorber layer includes a pattern representing a circuit image that may be transferred onto semiconductor wafers in a lithography system. As feature sizes of semiconductor devices decrease, the corresponding circuit images on the photomask also become smaller and more complex. Consequently, the quality of the mask has become one of the most crucial elements in establishing a robust and reliable semiconductor fabrication process.
In order to maintain the quality of the photomask throughout its lifetime, manufacturers have developed a pellicle to protect at least the patterned side of the photomask from being damaged by contaminants that may be present in semiconductor manufacturing tools. The pellicle typically includes a thin film attached to a frame, which has a height that places the thin film outside of the focal plane such that contaminants on the film are not imaged onto a semiconductor wafer.
Photomask contamination, however, may still occur under the pellicle film, as well as on the backside of the mask, during a lithography process. Organic and/or inorganic materials or chemicals used during the manufacturing processes may interact with photons to create airborne contaminants. Contaminants may also be created from material outgassing during lithography processing. Other sources of contaminants can include vaporized photoresist that is released during photoresist coating processes, evaporation of different chemicals during pre-bake and post-bake processes, solvents used on the photomask in during develop processes, and contaminants existing in the semiconductor fabrication facility (“Fab”) environment.
Contaminants may also enter into the space between the pellicle film and the photomask through, for example, vent holes in the pellicle frame. Additionally, contaminants may be trapped under the pellicle film after the pellicle is mounted on the photomask. The contaminants may create a haze in the optical path associated with the photomask assembly, which can reduce the clarity of the image in the focal plane. For example, a layer of contaminants may build up on the patterned side of a photomask, which may darken and distort an image projected onto a wafer.
Thus, detecting maze haze formation is critical to maintaining satisfactory wafer yield levels. To satisfy mask haze detection requirements, a Fab can perform thousands of mask scans with each inspection tool therein. However, current inspection tools detect mask haze only after the haze has developed into a precipitate, by which time the haze may already adversely affect imaging. Moreover, mask haze detection is often performed at regular intervals that are scheduled to minimize scanning resources while maximizing throughput, but the haze formation may develop into a precipitate more quickly than such intervals, thereby impacting wafer yield. In addition, conventional haze detection scanning requires that the entire mask must be scanned, thereby consuming a relatively large amount of time and decreasing product yield. Conventional haze detection tools and methods also require substantial initial and ongoing financial expenditures.